Assume for a second you are right (you aren't, it turns out)

Why does that matter?

IE what is the point you are trying to make by saying this?

Is it that they are self-interested?

Because if so, uh, almost all amicus briefs are filed by self-interested companies or advocacy groups.

That is in fact, one of the points of amicus briefs: To cover concerns not directly dealt with by the parties so that the court understands the broader implications of ruling one way or another.

Another point is to cover arguments the parties do not make.

The court is also free to deny them leave to file a brief if the court doesn't care.

I don't think this is true at all. Big companies can easily pay armies of lawyers to file patents on obvious ideas and then sue other companies. The ones taking a stand against this behavior don't want to do that, though. They know that implementation is the hard part in software engineering and would rather pay people to write software, even if that means some other company can "steal" their ideas.

I agree with the general notion that tech patents are too abstract, but you're right, its just too convenient for companies that don't hold many software patents to agree.

Now if a company with many software patents got behind this brief, I'd have more faith in the industry's desire to get rid of such patents.

Google has over 15,000 patents from their acquisition of Motorola mobile.

Mostly very specific wireless hardware patents, am I wrong?

To put it simply: Yes